Chem. Mater. 2010, 22, 5593–5600 5593 DOI:10.1021/cm101696t
Microcapsules with Tailored Nanostructures by Microphase Separation of Block Copolymers Jae Won Shim,† Shin-Hyun Kim,† Seog-Jin Jeon,†,§ Seung-Man Yang,*,† and Gi-Ra Yi*,‡ †
National Creative Research Initiative Center for Integrated Optofluidic Systems and Department of Chemical and Biomolecular Engineering, KAIST, Daejeon 305-701, Korea, and ‡Department of Industrial Engineering Chemistry, Chungbuk National University, Chungbuk, 361-763 Korea. §Current address: Material & Device Center, Samsung Advanced Institute of Technology, Yongin 449-712, Korea. Received June 17, 2010. Revised Manuscript Received August 29, 2010
Colloidal microcapsules of thin nanoshell membranes were prepared by the evaporation-induced self-assembly of poly(styrene)-block-poly(butadiene)-block-poly(styrene) (SBS) triblock copolymers inside the oil phase of a water-in-oil-in-water (W/O/W) double emulsion, which was prepared via a twostep sequential emulsification process. To see the nanoconfinement effect, the shell thickness was kept very thin and comparable to the feature scale of self-organized nanostructure. By exploiting the density mismatch between the microcapsules and the continuous medium, single-cored microcapsules with thin shells were selectively separated from mixtures of multicored microcapsules or thick-shelled microcapsules. The internal morphology of the membrane could be modulated by controlling the volume fractions of styrene and butadiene blocks, which was achieved simply by adding polystyrene homopolymer of relatively low molecular weight. By varying the amount of homopolymer, various morphologies were produced in the microcapsule membranes, including lamellae, perforated lamellae, and cylindrical and spherical phases. Moreover, the addition of polystyrene homopolymer could be used to control the mechanical characteristics of the microcapsules. Introduction Over the past few decades, supramolecular selfassembly has attracted considerable attention in nanofabrication as an alternative to expensive lithographic processes.1-3 In particular, block copolymer self-assembly has emerged as one of the most promising routes to nanostructures, because it can produce nanopatterns with structural complexity over large areas.4,5 By varying the block copolymer composition, segment-segment interaction parameters, and degree of polymerization, a variety of morphologies have been obtained and used for the nanopatterning of planar substrates consisting of organic, inorganic, semiconducting, metallic, or biologically relevant materials.6,7 Recently, the microphase separation of block copolymers has been achieved in nonplanar *Authors to whom correspondence should be addressed. E-mail:
[email protected] (S.M.Y.),
[email protected] (G.R.Y.).
(1) Ramos, L.; Lubensky, T. C.; Dan, N.; Nelson, P.; Weitz, D. A. Science 1999, 286, 2325–2328. (2) Yang, S.-M.; Jang, S. G.; Choi, D.-G.; Kim, S.; Yu, H. K. Small 2006, 2, 458–475. (3) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. J. Am. Chem. Soc. 2001, 123, 8718–8729. (4) Bates, F. S. Science 1991, 251, 898–905. (5) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735–738. (6) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. Adv. Mater. 2006, 18, 2505–2521. (7) Wang, Y.; Hong, X.; Liu, B.; Ma, C.; Zhang, C. Macromolecules 2008, 41, 5799–5808. (8) Shin, K.; Xiang, H.; Moon, S. I.; Kim, T.; McCarthy, T. J.; Russell, T. P. Science 2004, 306, 76. (9) Jeon, S.-J.; Yi, G.-R.; Koo, C. M.; Yang, S.-M. Macromolecules 2007, 40, 8430–8439. r 2010 American Chemical Society
confining geometries such as cylindrical pores8 and emulsion droplets.9,10 Depending on the surface preference and the size of the confining geometry relative to the characteristic length of the microphase, unique and anomalous morphologies have been observed in both experimental and theoretical studies.11-15 Meanwhile, encapsulation technologies have long been a topic of interest, because of their wide range of applications in foods, drugs, and cosmetics.16-19 Emulsion droplets have been used as templates for preparing spherical microparticles to encapsulate active materials. In addition, double-emulsion droplets have been employed to generate microcapsules containing liquid cores,20-22 in (10) Yabu, H.; Higuchi, T.; Shimomura, M. Adv. Mater. 2005, 17, 2062– 2065. (11) Jeon, S.-J.; Yi, G.-R.; Yang, S.-M. Adv. Mater. 2008, 20, 4103– 4108. (12) Tanaka, T.; Saito, N.; Okubo, M. Macromolecules 2009, 42, 7423– 7429. (13) Higuchi, T.; Tajima, A.; Motoyoshi, K.; Yabu, H.; Shimonura, M. Angew. Chem., Int. Ed. 2008, 47, 8044–80146. (14) Okubo, M.; Saito, N.; Takekoh, R.; Kobayashi, H. Polymer 2005, 46, 1151–1156. (15) Pinna, M.; Guo, X.; Zvelindovsky, A. V. Polymer 2008, 49, 2797– 2800. (16) (a) Dubey, R.; Shami, T. C.; Bhasker Rao, K. U. Defense Sci. J. 2009, 59, 82–95. (b) Degim, I. T.; Celebi, N. Curr. Pharm. Des. 2007, 13, 99–117. (17) Davis, S. S.; Walker, I. M. Methods Enzymol. 1987, 1749, 51–64. (18) Tjipto, E.; Cadwell, K. D.; Quinn, J. F.; Johnston, A. P. R.; Abbott, N. L.; Caruso, F. Nano Lett. 2006, 6, 2243–2248. (19) Becker, A. L.; Zelikin, A. N.; Johnston, A. P. R.; Caruso, F. Langmuir 2009, 25, 14079–14085. (20) Pannacci, N.; Bruus, H.; Bartolo, D.; Etchart, I.; Lockhart, T.; Hennequin, Y. Phys. Rev. Lett. 2008, 101, 164502.
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Figure 1. (a) Schematic illustration for preparation of microcapsules with nanostructure, based on microphase separation of block copolymers in the middle phase of the double-emulsion droplets. (b, c) Confocal laser scanning microscopy (CLSM) images of microcapsules encapsulating FITC-labeled dextran, where shells are doped with Nile-Red. The scale bars in panels (b) and (c) are each 20 μm.
which the middle phase of the double emulsion droplets is selectively solidified by polymerization23-26 or evaporation-induced consolidation.21,22,27,28 For example, optofluidic encapsulation can be used to confine an aqueous suspension containing a crystalline colloidal array within a spherical membrane through in situ photopolymerization of the middle phase.26 However, most of the previous microcapsules composed of a solid shell do not have any nanostructures in the membrane. Although a few recent papers reported microcapsules of nanopores or micropores, the regularity of the pores was not good enough to facilitate controlled mass transfer through the membrane.27,28 On the other hand, amphiphilic block copolymers have been explored in studies of so-called “polymersomes”, which show enhanced stability and controlled permeability compared with liposomes.21,22 However, the bilayered membranes behave like a semipermeable membrane that allows only very small molecules to pass through it. In the present study, we prepared polymeric microcapsules of nanostructured membrane using symmetric (21) Shum, H. C.; Kim, J.-W.; Weitz, D. A. J. Am. Chem. Soc. 2008, 130, 9543–9549. (22) Hayward, R. C.; Utada, A. S.; Dan, N.; Weitz, D. A. Langmuir 2006, 22, 4457–4461. (23) Hennequin, Y.; Pannacci, N.; Torres, C.; T.-Meris, G.; Chapuliot, S.; Bouchaud, E.; Tabeling, P. Langmuir 2009, 25, 7857–7861. (24) Nie, Z.; Xu, S.; Seo, M.; Lewis, P. C.; Kumacheva, E. J. Am. Chem. Soc. 2005, 127, 8058–8063. (25) Kim, J.-W.; Utada, A. S.; Fernandez-Nieves, A.; Hu, Z.; Weitz, D. A. Angew. Chem., Int. Ed. 2007, 46, 1819–1822. (26) Kim, S.-H.; Jeon, S.-J.; Yang, S.-M. . J. Am. Chem. Soc. 2008, 130, 6040–6046. (27) Lee, D.; Weitz, D. A. Adv. Mater. 2008, 20, 3498–3503. (28) Choi, S.-W.; Zhang, Y.; Xia, Y. Adv. Funct. Mater. 2009, 19, 2943– 2949.
poly(styrene)-block-poly(butadiene)-block-poly(styrene) (SBS) triblock copolymers, which were confined in the oil phase of water-in-oil-in-water (W/O/W) double-emulsion droplets. The microphase separation of the block copolymer created an ordered nanostructure in the thin membrane of microcapsules, which is potentially useful in the preparation of regular nanopores for controlled mass transfer through a membrane. Here, the shell membranes were made nanoscopically thin, to see the confinement effect on the phase morphology. Because the double-emulsion droplets were prepared by sequential emulsification, their size distribution and the number of cores in each droplet could not be controlled precisely.29 However, we could separate high-quality microcapsules through a simple separation scheme, based on selective sedimentation after consolidation of the middle phase. In addition, the internal nanoscopic morphology resulting from microphase separation of the block copolymers was investigated by slicing individual microcapsules with an ultramicrotome and examining the internal structure using transmission electron microscopy (TEM) analysis. The nanostructure of the block copolymers inside the microcapsule shells was modulated precisely by adding PS homopolymer (hPS) with relatively low molecular weight, and the mechanical properties of the microcapsules varied, depending on the block copolymer composition. In the subsequent sections, we first described the method for preparing microcapsules with thin block copolymer shells and the selective separation process. We then examined the microphase separation of block copolymers (29) Ikkai, F. Langmuir 2008, 24, 3412–3416.
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confined in the middle phase of double-emulsion droplets. Finally, we discussed the effect of hPS on the morphology of the block copolymers as well as the mechanical properties of the microcapsules. Results and Discussion In this work, two-step sequential emulsification was used to prepare double-emulsion droplets, which is the only route to the mass production of such droplets at low cost. In the first emulsification, water-in-oil (W/O) droplets were prepared in which the oil phase was comprised of SBS triblock copolymers and hPS in a mixture of dichloromethane (DCM) and toluene. In subsequent emulsification, the W/O emulsion droplets were emulsified into aqueous solution containing oil-stabilizing surfactant using a homogenizer, resulting in a polydisperse mixture of simple (oil-in-water), single-cored double, and multicored double emulsion droplets. Thus, by evaporation of volatile organic solvents, those emulsions were solidified, which produced polymeric microspheres, or microcapsules with single cores or multicores. To separate the single-cored microcapsules from the mixture, we used a selective sedimentation process that separated microcapsules on the basis of their average density. When the density ratio (γ = Fcapsule/Fcontinuous) of microcapsules to continuous phase is >1, microcapsules should settle slowly. The density of microcapsules can be determined by the following equation: Fcapsule ¼ φcore Fcore þ φshell Fshell where φcore and φshell are volume fractions of core and shell inside microcapsules, respectively. When the densities of the core (Fcore), continuous phase (Fcontinuous), and shell (Fshell) are in the order Fcore > Fcontinuous > Fshell, the single-cored microcapsules with shells can be settled in a case when a shell volume is sufficiently small enough to make the average density of the capsules (Fcapsule) higher than Fcontinuous. Otherwise, polymeric capsules or particles should be creamed up. From a simple consideration of average density, the ratio of critical shell thickness to microcapsule radius can be determined for γ > 1: !1=3 tcritical Fcore - Fshell ¼ -1 ð1Þ R Fcontinuous - Fshell where Fshell is determined by the fraction of polystyrene (PS) and polybutadiene (PB) chains in the polymer blend. Therefore, through the sedimentation process, we could obtain microcapsules with thin shells from bulk mixtures without the need for complex and inefficient procedures such as sieving. For instance, when microcapsules have thin shells with lower density but cores with higher density than the continuous phase, they settle slowly. By contrast, multicored microcapsules or thickshelled capsules float on top of the continuous phase. Therefore, we can selectively separate single-cored microcapsules with thin shells by controlling the density of the continuous phase.
Figure 2. (a) Optical microscopy image of microcapsules with fs = 0.31 dispersed in water. (b) Size distribution of the microcapsules based on panel (a). (c) SEM images of dried microcapsules with fs = 0.31. All capsules were completely deflated during drying due to high flexibility of the membrane. (d) TEM images taken at the central part of deflated microcapsules where two line patterns from two membranes were overlapped. The inset in panel (d) is a TEM image of a deflated capsule at low magnification. (e, f) TEM images showing a cross section of a shell membrane of the microcapsule. The inset of panel (f) is a schematic of the prepared microcapsules with cylindrical microphase in shell. The scale bars in panels (a) and (c) and panels (d, e, f) are 100 μm and 200 nm, respectively.
To confirm the successful separation of microcapsules, and to identify their core-shell structure, we dissolved fluorescein isothiocyanate (FITC)-labeled dextran and Nile-Red dye molecules in the core and middle phases, respectively. After complete evaporation of the middle phase and separation, the microcapsules were observed by confocal laser scanning microscopy (CLSM), in which the aqueous dextran cores are labeled with FITC and the polymeric shells are doped with Nile-Red (see Figures 1b and 1c). The images show uniform thickness of the membrane, which contributes to the uniform oil layer in double-emulsion droplets. A large resistance of lubrication film, highly thin oil layer, against a gravitational force on core droplet makes relatively concentric core and shell droplets. Inside the thin shells of the microcapsules, the triblock copolymers self-organized into ordered nanostructures by thermal annealing at 95 °C, as schematically depicted in Figure 1a. The interface between the middle and continuous phases in each doubleemulsion droplet is stabilized with the block copolymer surfactant poly(ethylene oxide)-block-poly(propylene oxide)block-poly(ethylene oxide) (PEO-b-PPO-b-PEO, Pluronic F108, BASF).
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Microcapsules with a Cylindrical PS Phase. Block copolymers inside the thin-shelled microcapsules can form various nanoscale morphologies through microphase separation, depending on the volume fraction of the styrene (or butadiene) block. In this study, we used blends of block copolymer with hPS to control the volume fraction instead of block copolymers with different molecular weight ratios.9,11 As a starting point, we prepared microcapsules composed of SBS triblock copolymers with a PS weight fraction (fs) of 0.31. Because the block copolymer of PB (FPB = 0.892 g/cm3) and PS (FPS = 1.05 g/cm3) with fs = 0.31 has an average density of 0.941 g/cm3, we used a 3.85 wt % aqueous solution of gelatin (Fcore = 1.013 g/cm3) as the core liquid and 0.1 wt % aqueous solution of PEO-b-PPO-b-PEO (Fcontinuous= 1.0003 g/cm3) as the outer continuous medium, in which microcapsules with thin shells (tcritical < 1 μm) were successfully separated by spontaneous sedimentation. The sequential emulsification and subsequent evaporation of the volatile organic phase produced a mixture of various types of microspheres; this is demonstrated in the inverted optical microscopy image in Figure S1 in the Supporting Information, in which the transparent microspheres are single-cored microcapsules and the opaque or relatively small spheres are multicolored microcapsules or microparticles. Notably, the transparent microcapsules are larger than the other microspheres. The size distributions of the transparent and opaque spheres are shown in Figure S1b in the Supporting Information. Through an acceleration of the sedimentation-induced separation by centrifugation at 8500 rpm for 10 min, we could collect the microcapsules from the mixture as shown in Figure 2a. Many transparent microcapsules show color at their edges under reflection-mode optical microscopy, because of optical interference induced by the thin membrane.29 The size distribution of the microcapsules is shown in Figure 2b, where the microcapsule average diameter and coefficient of variation (CV) are 25 μm and 0.328, respectively. According to eq 1, we can estimate tcritical/R as 0.067 and, therefore, the separated microcapsules will have a thin shell with a thickness of
